Heat transfer mechanisms are broadly classified into heat conduction, heat convection and heat radiation. Infrared heating is based on heat radiation and does not need a thermal medium. Accordingly, an energy transfer efficiency achievable by infrared heating based on heat radiation is higher than that of the other heating based on heat conduction or heat convection. Infrared heating is therefore widely utilized in many areas including, machinery, metal, chemical, electrical, electronics, printing, food processing and medical. For example, infrared heating is used for bake-coating of a car body, curing of a thermoplastic resin, defreezing of a frozen food, heating of a room, and medical treatment of a human body.
The above-mentioned infrared heating is carried out by means of an infrared radiator for converting electric energy into heat radiation energy.
One of such infrared radiators is disclosed in the "New Materials Handbook", page 69, published by the Trade and Industry Survey Association in Japan on Jan. 5, 1986 (hereinafter referred to as the "prior art"). An infrared radiator 1 of the prior art is described below with reference to FIG. 1.
FIG. 1 is a schematic vertical sectional view illustrating the infrared radiator 1 of the prior art, as used as a room heating device. As shown in FIG. 1, the infrared radiator 1 of the prior art comprises a hollow casing 2 made of ceramics, and a nichrome wire (i.e., a nickel-chromium alloy wire) 3, as a resistance heating element, provided in the casing 2.
By causing electric current to flow through the nichrome wire 3 of the infrared radiator 1 of the prior art, the nichrome wire 3 generates heat, and as a result, the casing 2 made of ceramics emits the infrared rays.
However, the above-mentioned infrared radiator 1 of the prior art has the following defects:
(1) Difficulty in forming of ceramics restricts the size of the casing 2, thus making it impossible to manufacture a large-sized infrared radiator 1. It is therefore necessary, when heating a large room, to use a plurality of infrared radiators 1, requiring a higher cost.
(2) There is a gap between the casing 2 and the nichrome wire 3 as the resistance heating element, resulting in a large loss of heat transfer from the nichrome wire 3 to the casing 2, and hence in a low infrared radiation efficiency.
(3) The casing 2 made of ceramics is brittle and tends to easily break, thus requiring considerable care in handling.
(4) The nichrome wire 3 as the resistance heating element is easy to break, resulting in a short service life.
Another infrared radiator is proposed, which comprises a plate made of ceramics and a nichrome wire as a resistance heating element, stuck onto one surface of the plate. This infrared radiator, not having the defect (2) of the above-mentioned infrared radiator 1 of the prior art, has the other defects (1), (3) and (4), and in addition, the following defect:
Because of a considerable difference in thermal expansion coefficient between the plate made of ceramics and the nichrome wire as the resistance heating element, a serious thermal strain is produced during service, resulting in a peeloff occurring on the interface between the plate and the nichrome wire and an easy occurrence of cracks in the plate or breakage of the nichrome wire. This another infrared radiator cannot therefore withstand repeated use for a long period of time.
Under such circumstances, there is a strong demand for the development of an infrared radiator which is not subject to restrictions in size in the manufacture thereof, has a high infrared radiation efficiency, is hard to break, can withstand repeated use for a long period of time, and can be efficiently and economically manufactured, but such an infrared radiator has not as yet been proposed.